The hundreds of theatergoers who packed into Central London's historic Apollo Theatre on Thursday night were expecting to watch a mystery.

But about 40 minutes into the play, shortly after 8 p.m., they got a drama instead.

"One of the actors said, 'Watch out!'" said one woman. "We thought it was part of the play."

It wasn't. Instead, it was part of the century-old structure's ornate plaster ceiling, which tumbled five stories onto the theatergoers, injuring scores of them, seven seriously, officials said.

"We heard a creak, somebody screamed, somebody from over there said, 'Look out!' and then the ceiling kind of creased in the middle and then just collapsed," said Hannah, who with her husband had snagged one of the last pairs of tickets to the performance in the 775-seat theater.

Though she said she herself felt "slight panic," she credited those around her for keeping their calm. "It wasn't every man for himself," she said, noting that several of her fellow theatergoers checked on those around them. "Everyone was looking out for each other, and in a couple of minutes, everyone was out."

The London Fire Brigade's Kingsland Station Manager Nick Harding said about 720 people were inside when a section of the ceiling collapsed on the theatergoers, taking parts of the balconies with it.

London theater collapse 18 photos

London theater collapse18 photos

London theater collapse – Emergency service personnel work outside London's Apollo Theatre on Thursday, December 19. Part of the theater's ceiling collapsed during a performance Thursday night, injuring dozens of people, officials said.

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London theater collapse18 photos

London theater collapse – Firefighters confer at the scene. Nick Harding, the London Fire Brigade's Kingsland Station manager, said about 720 people were inside the theater when a section of the ceiling collapsed, taking parts of the balconies with it.

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London theater collapse18 photos

London theater collapse – Paramedics attend to an injured person at the scene of the collapse.

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London theater collapse18 photos

London theater collapse – A hydraulic lift is used to investigate the cause of the collapse. "We heard a creak, somebody screamed, somebody from over there said, 'Look out!' and then the ceiling kind of creased in the middle and then just collapsed," one theatergoer said.

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London theater collapse18 photos

London theater collapse – An injured woman cries inside an ambulance.

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London theater collapse18 photos

London theater collapse – People sit aboard a bus used to hold theatergoers.

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London theater collapse18 photos

London theater collapse – An injured woman is taken toward a waiting ambulance.

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London theater collapse – The Apollo Theatre is located next to Piccadilly Circus in the heart of London. The ceiling collapsed during a packed performance of "The Curious Incident of the Dog in the Night-Time."

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London theater collapse18 photos

London theater collapse – A man wraps himself in an emergency foil blanket provided by service workers. One witness said the collapse "was like slow motion." Another said, "You initially thought it was part of the play, and then you could feel things on you."

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London theater collapse – Emergency vehicles line the street near the theater. The Apollo Theatre was built in 1901 and holds 775 seats on four levels, according to its website.

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London theater collapse – An emergency worker pushes a stretcher along Shaftesbury Avenue.

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London theater collapse – Injured people sit on a bus used for theatergoers.

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London theater collapse – Metropolitan Police said in a tweet that those who were seriously hurt had been taken to hospitals in central London.

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London theater collapse18 photos

London theater collapse – A woman wears bandages and a blanket outside the theater.

Actors on stage yelled, 'Watch out!'

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All evacuated from London Apollo Theatre

An ambulance official said a total of 76 people had been injured, though many of them were treated at the scene and released.

"It was like slow motion," said one girl, who initially thought the sound of the falling ceiling was sound effects, but soon learned otherwise. "It just kinda came down."

Harding said what fell was a 10-meter-by-10-meter square section of the ceiling, and that it dragged down balconies with it.

"You initially thought it was part of the play and then you could feel things on you," said an older woman. "The dust that came down -- you couldn't see in front of you."

Metropolitan Police said those who were seriously hurt had been taken to hospitals in central London. None of the injuries appeared life threatening, one official said.

Within an hour, a spokesman for the fire department said everyone, including those who had been hurt, had been evacuated from the building.

Martin Bostock, who was with his wife and two children, said he thought the cave-in was part of the show until something very hard hit him on the head and chaos and panic erupted.

"You couldn't see across the room because of the dust, which we were all breathing in," he told CNN. "It was absolutely horrific and very terrifying. I was with my wife and two kids. Thank God, we got out."

The Apollo is located next to Piccadilly Circus in the Soho district, which is usually packed with tourists, shoppers and diners at theater time.

Within minutes, paramedics arrived carrying stretchers as police cordoned off the area.

A few minutes later, some of the paramedics emerged from the theater, their stretcher full; others helped someone limp out of the building.

Many of the injured were taken initially to the nearby theaters for an initial assessment of their condition by medical workers.

The initial police assessment of the collapse was that it was an accident. "There is no suggestion at this stage that this was as a result of a criminal act, however, at this stage we are keeping an open mind," Metropolitan Police said in a statement.

Outside the Apollo, the street was packed with dozens of police, fire and ambulance vehicles as well as well-dressed theatergoers, some of their faces darkened with dust.

"The Curious Incident of the Dog in the Night-Time," which was adapted to the theater from the 2003 mystery novel by British writer Mark Haddon, is about a 15-year-old boy's investigation into the death of a neighborhood dog.

Ticket prices at the four-level theater, which opened its doors in 1901, included a £1 ($1.64) "theatre restoration levy," the theater's website said. It was not immediately clear whether the ceiling was part of the restoration of the building, which is owned and operated by Nimax Theatres.

In a statement, a spokesman said the company would "continue to cooperate fully with the authorities to establish exactly what happened tonight."

Prime Minister David Cameron, in a tweet, commended the fast work of emergency services.

Now, what started as a mystery and veered into drama has again become a mystery, as engineers try to determine what caused the collapse.